The present invention relates to a distance-controlled tunneling transducer for use in a direct access storage unit, having a plurality of tunnel tips arranged facing a recording medium. In particular, the invention teaches improved gap control means for implementation in micromechanical techniques. The invention is also applicable to low-voltage field-emission environments where the gap dimension is somewhat larger than in the tunneling regime. Therefore, where in the following description reference is made to tunneling phenomena, those skilled in the art will be able to easily apply what is said to field-emission phenomena as well.
In the tunneling regime, the tip/surface distance typically is less than 2 nm, and in the field-emission environment, that distance is considered to be on the order of 20 nm. Small local deviations from planarity of the surface of the recording medium, say on the order of tenths of a nanometer, may result in relatively large changes of the tip current, in particular in the tunnel regime, where the dependence of the tunneling current on the tip-to-surface distance is exponential. Because of the fact that the operating distances in tunneling as well as in field-emission environments are so small, it may even happen that the tip crashes into a surface asperity and thus suffers damage, unless some measure is taken to maintain the tip-to-surface distance essentially constant. In conventional tunneling microscopes, this problem is solved by means of a feedback loop operating from the tip distance above the surface, with the aim of keeping the tip current constant.
It will be obvious to those skilled in the art that in view of the relatively large velocity with which surface asperities may be encountered as the tip scans across the surface of the recording medium, and because of the possibly abrupt change in tip-to-surface distance, a very fast response of the feedback loop is required.
Recently, direct access storage devices have been proposed which operate on the principle of the scanning tunneling microscope. The basic reference in this area is EP A-0 247 219. The reference teaches a storage unit comprising an array of tunnel tips arranged at a tunnel distance from the surface of a recording medium which is capable of permitting digital information to be written or read through variations of the tunneling current. The tunnel distance is maintained by means of a feedback loop, and the idea of integrating the control circuitry of that feedback loop on the tunnel tip array is mentioned. No details of the circuitry nor of the way the integration can be achieved are, however, given.
A scanning tunneling microscope realized in micromechanical techniques is disclosed in U.S. Pat. No. 4,520,570. A semiconductor chip has slots etched into it in a pattern resulting in a plurality of tunnel tips to be formed that are hinged by stripes of semiconductor material to the main body of the chip. Again in this reference, an area is provided on the semiconductor chip to contain control circuitry associated with the tunnel tip, in casu a transistor acting as an impedance transformer.
While it is acknowledged that the idea has occurred to integrate the tunnel tip feedback loop into the semiconductor chip on which the tunnel tip is formed, it has turned out that conventional control circuitry for scanning tunneling microscope and field-emission microscope applications is by far too complex and, hence, too bulky to be installed on the semiconductor chip if the chip is to carry a plurality of tips arranged in a small array.
As a possible alternative, one may require that both the array of tips and the surface of the recording medium be sufficiently flat, for instance within 0.1 nm over the area to be scanned, to allow for global gap width control averaging over the currents of all tips. Such a requirement would, however, impose undesirable constraints with regard to precision of manufacturing and alignment, as well as to the choice of materials for the recording medium.